Mid-bus test equipment, such as analyzers and jammers, are connected in the middle of a bus between devices that are communicating with each other. For example, protocol analyzers are used to capture, examine, and debug the complex protocols used by storage and networking busses. Jammers, which are also called error injectors, are used to modify a transmission in order to test for proper recovery from errors, or to test for proper handling of unusual protocol sequences. Modern point-to-point busses transfer gigabytes per second on each differential transmission line of a network bus or a storage bus. For a bus that operates point-to-point, it is often difficult to tap into the bus to connect mid-bus test equipment without degrading signal quality of transmissions over the bus.
Tapping into a transmission line of a bus degrades transmitted signals because the tapping can redirect a portion of the signal and can also change the impedance of the transmission line, creating signal reflections. As an alternative to tapping into a transmission line, mid-bus test equipment often use a technique of terminating the received signal and then retransmitting it again as a clean new point-to-point signal with full strength.
One example of a point-to-point bus is a serial attached small computer standard interface (SAS). SAS uses an equalization protocol called transmitter training (Tx Training) that allows the receiver of the signal to adjust the equalization parameters or coefficients used by the transmitter, thus optimizing the transmitter frequency profile. Tx Training allows the receiver to compensate for the frequency profile of the transmission line resulting in a much cleaner signal at the receiver.